Cannot create a new milestone inside a project

Hi, I am new to Gitlab.

Inside an imported project from github, when I try to create a new milestone, I get an error 500 when I click on “Create Milestone”. Here is the message:

500 Whoops, something went wrong on our end.

** Context ** To give more context, my project was imported from github with more than 50 milestones.

It looks like an import problem, since I manage to create milestones in other projects and group.

I am stucked !

Any idea what’s going wrong ?

Thank you very much for your help,